As will be developed in greater detail below, low-methane naturally occurring gases, frequently called mine gases, can be utilized to produce a product gas having a higher methane concentration by pressure-swing adsorber processes utilizing a plurality of adsorbers in which the adsorbent can be a carbon-based material tending selectively to adsorb methane at an elevated pressure and to desorb methane upon pressure reduction.
In such processes, the naturally occurring low-methane gas, which can be referred to as a mine gas or fire damp, is passed through an adsorber in which the adsorbent selectively adsorbs methane, the adsorbed methane is then released by dropping the pressure and the methane-rich product gas is then discharged from each adsorber.
To protect mine shafts or galleries and, in general, deep coal mining operations, especially bituminous coal mining, against the incursion of fire damp or the development of explosive conditions because of the buildup of methane in the mine atmosphere, it is known to provide bores or holes in the rock structure around the galleries, tunnels or shafts from which methane-containing gases are evacuated by suction pumps or the like. These subterranean gases, which can be described as methane-poor or low-methane gases, can be referred to for convenience as mine gases and generally contain between 25 and 45 volume percent methane together with other components such as nitrogen, oxygen and carbon dioxide.
Such gases are analogous to natural gas but, in spite of the significant methane concentration, do not contain sufficient methane for direct use as a product gas, and are also characterized by a high degree of variability in the methane concentration of the extracted gases.
The subterranean extraction of gases in this manner reduces the tendency of methane incursion into the mine shafts or galleries and thus plays a protective role.
The methane content of the recovered gases, generally above 25% by volume, is well above the upper ignition point (14 volume percent methane) although the heat value of the extracted mine gas is comparatively small, especially when, as is usually the case, the mine gas contains less than 35% by volume methane, a level below which the gas cannot practically be used as a fuel for economical reasons.
As a consequence, it is desirable to increase the methane content of the mine gas so that its heat value is substantially equal to that of the natural gas and other gases utilized for municipal heating and other purposes. To this end it is desirable to so enrich the gas in methane with the methane content will be about 50% by volume.
One obvious way to enrich the mine gas to this concentration of methane is to add liquefied methane to it. This is expensive because the production of liquefied methane is costly and hence this approach has not been utilized economically to any great extent.
In U.S. Pat. No. 4,305,734, there is described a pressure-swing adsorption method of enriching the gas with methane.